Thermo-mechanical treatment of zirconium alloys

ABSTRACT

A zirconium alloy comprising at least 95 percent zirconium, which has been thoroughly annealed, is greatly increased in strength without substantial loss in ductility by subjecting it to tensile creep deformation in a temperature range in which creep will occur, yet which is below the temperature for significant recovery.

United States Patent [1 1 Levy 1 1 THERMO-MECHANICAL TREATMENT OF ZIRCONIUM ALLOYS [75] Inventor: Ira S. Levy, Kennewick, Wash.

[73] Assignee: Exxon Nuclear Company Inc.,

Bellevue. Wash.

[22] Filed: Feb. 26, 1973 [21] Appl. No.: 335,679

[52] U.S. CI. l48/Il.5 F; 72/364; 72/378; 148/133 [51] Int. Cl. C22f 1/18 [58] Field of Search 148/11.5 F, 12.7, 131, 148/133, 32, 32.5; 72/364, 378

[56] References Cited UNITED STATES PATENTS 2,836,527 5/1958 Kessler et al. 148/131 X [451 May 20, 1975 3,201,287 8/1965 Flowers 148/131 X 3,337,372 8/1967 Reed-Hill 148/131 X 3,567,522 3/1971 Thomas et a1. 148/1 1.5

OTHER PUBLICATIONS Applications-Related Phenomena in Zirconium and Its Alloys, Amer. Soc. for Testing Mtls., Phila., Pa., Nov. 1968. pp. 210-225.

Primary ExaminerC. Lovell [57] ABSTRACT THERMO-MECHANICAL TREATMENT OF ZIRCONIUM ALLOYS BACKGROUND OF THE INVENTION Zirconium is important as a structural material in the nuclear industry because of its low absorption cross section for thermal neutrons. In order to improve its mechanical properties, it is usually alloyed with one or more metals selected from the group consisting of chromium, tin, niobium, iron, tantalum, nickel and aluminum, with the zirconium constituting at least 95 percent by weight.

The zircaloys are a particularly important group of zirconium alloys. They contain at least 97.5 percent by weight zirconium and small amounts of tin. Usually, iron is also present. The alloy which is most widely used at present is zircaloy-2. Zircaloy-2 is an important structural material in the nuclear field because of its low absorption cross section for thermal neutrons and its good mechanical properties. It is of particular importance as the cladding for nuclear fuel elements. For this purpose it is commonly used in the form of tubes which are filled with a nuclear fuel material, usually uranium oxide or a mixture of uranium and plutonium oxides.

Upon irradiation in a nuclear reactor, there may be swelling of the nuclear fuel. It is very important to avoid bursting of the zircaloy tube since this would permit the release of the highly radioactive fission products into the cooling fluid. It is therefore desirable that the zircaloy have both high ductilityand high strength under the operating conditions. It is also desirable for the zircaloy to have good resistance to corrosion by the cooling fluid, which in pressurized water reactors is high temperature water and in boiling water reactors a mixture of high temperature steam and water.

The prior practice was to produce tubing by combinations of cold work and intermediate temperature annealing. This gave a tubing which had high strength but relatively low ductility and relatively poor resistance to high temperature corrosion as compared to annealed material. One reason for desiring high ductility is the fact that neutron irradiation causes hardening of the metal. While this increases the strength, it also produces embrittlement. If the material has initially high ductility, it will tend to be less brittle after irradiation.

In the prior art, it was necessary to choose between good ductility and corrosion resistance on the one hand and high strength on the other hand. The object of this invention is to obtain a combination of high strength, high ductility and good corrosion resistance.

Zircaloy-Z has the following composition in weight percent:

Sn 1.20 1.70 Fe 0.07 0.20 Cr 0.05 O.l5 Ni 0.03 0.08 Fe+Cr+Ni 0.18-0.38 N max. 0.10

N average 0.008

0 average 0.17

Zr Balance It has an 0: (alpha) a ,8 (beta) transformation point of about 940C and an a +3 B transformation point of about 1,000C.

Next to zircaloy-2 the most important zircaloy is zircaloy-4 which differs from zircaloy-2 is being substantially free of nickel. A typical composition in weight percent is:

Ni 0.0023 Zr Balance Zircaloys -l and -3 have the following typical compositions in weight percent:

SUMMARY OF THE INVENTION I have devised a thermomechanical treatment (TMT) of the zirconium alloys mentioned above which gives a product having high strength, high ductility and good corrosion resistance to high pressure steam at 400500C. It involves thorough annealing followed by tensile creep deformation of 3 to 10 percent in a temperature range in which creep will occur, yet which is below the temperature of significant re overy, i.e., the temperature at which significant annealing out of the dislocations produced by the deformations would occur. For the zircaloys, the temperature range is 200 to 350C. It can be applied as are-treatment to commercially supplied tubing with only very slight changes in diameter and wall thickness. By the term tensile creep deformationl'l mean a tensile deformation in which at least the final portion is carried out under a substantiallyconstant load and at a comparatively low strain rate which may be, e.g., 6 X 10 inch/inch/minute.

From electron microscope studies, it appears that the strengthening is related to the mutual interaction of a uniform, complex dislocation array produced by the treatment and possibly by the interaction of dislocations with fine precipitates.

The mechanism for the retention of high ductility with strengthening is uncertain. It may be related to the nature of the dislocation array.

DETAILED DESCRIPTION The detailed description will be directed to the treatment of zircaloy. The starting material, which may have received prior working and heat treatment of varying types and may exhibit dislocation structure, is heated to a temperature in the upper part of the alpha range, preferably 650850C, and is held at that temperature until a thoroughly annealed condition is produced. For example, holding at a temperature of 750C for 1 hour is satisfactory.

It is then subjected to a tensile creep deformation of 3 to 10 percent. The deformation process involves tensile loading, at a temperature of 200-350C, to the desired strain at a strain rate of about 0.001 to 0.01 inch/inch/minute followed by holding the material at the final load and under the above temperature for a period of at least 15 minutes. During the hold period a small amount of additional creep takes place and the dislocation array attains its final form. In the upper portions of the deformation range, the strain should be applied Except as indicated below, all anneal, deformation and aging procedures were carried out in air. During the deformation step, the specimens were heated in a 4-inch long quartz heater to give a uniform 2-inch long in increments with intermediate cooling, e.g., several heated zone. Tensile loads were applied on an Instron successive elongations of 3 percent, each followed by tensile machine through friction grips attached to the a hold period as defined above. However, the presently cold ends of the tubes projecting from the furnace. preferred embodiment involves a single elongation of Specimens were deformed at a crosshead speed of 0.05 about 5 percent at a temperature of about 225C. inch/minute to a desired strain level, then they were Between the annealing and deformation steps the maintained at that load for a prescribed length of time material is ordinarily allowed to cool in air to room during which a small amount of creep occurred. temperature, but it may be cooled by circulating gas, as Thermomechanical treatment, consisting of tensile Will be described later in more detail creep deformation at intermediate temperatures, was first a lied to the as-received tubing. In some cases EXAMPLE I this WES followed by intermediate temperature aging. A series of es s was r n n i ytubing as Tensile tests were carried out at 300C. Results are ceived from a vendor. It had a specified inner diameter Shown i T bl I of 0.472 i 0.002 inch and a wall thickness of 0.035 i A ill b Seen f T bl I, treatment under those 00026 ineh- According to the Vendor, it had been conditions produced no advantageous changes. Only formed by a series of cold working steps with interme- 0 h 5 percent d f ti wa a lied at 300C diate anneals, ending with a final stress relief treatment (specimens L and O) was the strength as high as that Of 4 hours at 910F Specifications for this of the as-received material. Moreover, specimen L yp of tubing can for a final Stress relief at showed reduced ductility and specimen 0 showed ther- 7800-1900013 3- Speeimens Were thinned mal instability. In no case was there any improvement by electro-polishing and transmission electron microi d ili graphs Were made- The mierostruetllfe consisted of Next, specimens were annealed at temperatures of both large, clear grains, yp of material that has 650l,0l0C, withdrawn from the furnace, allowed to been Partially recrystallized, as Well as of grains cool in air, then subjected to tensile creep deformation ing typical deformation structure, the latter con- 200 to 300C l h TMT)or 1,010C (beta TMT).

stituting about percent of the microstructure. There were some precipitates, which were not identified, within the recrystallized grains.

The tubing was cut into 67 inch lengths for the various treatments and tests.

In the beta TMT, the tubes were enclosed in a metal chamber loaded through an O-ring seal with the interior under a continuous vacuum of 10 microns.

Results are shown in Table II. Tensile properties were again measured at 300C.

TABLE I TENSILE PROPERTIES AT 300C FOR TMT OF AS RECEIVED TUBING Tensile Properties" Speci- Deformation Parameters Aging Parameters Total men Temperature Hold Time Temperature Time 0.2% YS UTS Elongation No. "C Elongation Minutes C Hour lbs lbs 7:

A As-received Control 3250 3870 24.7 B As-received Control 3250 3960 26.0 C As--received Control 3255 3760 24.7 D 480 5.0 4.5 Broke during deformation E 480 3.0 10 Broke during deformation F 480 0.66 15 1900 2560 25.3 G 480 1.0 15 2100 2750 24.7 H 480 1.67 15 450 1 1650 2200 26.0 J 300 3.0 15 2500 3770 22.7 K 300 1.0 15 2450 3600 23.7 I. 300 5.0 15 3550 3720 19.0 M 300 3.0 15 300 l 2750 3760 23.3 0 300 5.0 15 300 1 3250 3680 23.3 P 300 3.0 15 450 1 2350 2950 21.0 0 300 5.0 15 450 1 2380 2880 22.5

"Yield and ultimate strength were measured on1 as load tlhs) during screening tests (Tables 1 and 11) since tubing dimensions were fairly uniform.

TABLE II TENSILE PROPERTIES AT 300C FOR TMT OF ANNEALED TUBING Tensile Properties Speci- Anncal Parameters Deformation Parameters Total men Temperature Time Temperature Hold Time 0.29! YS UTS Elongation No. I Hour C Elongation Minutes lbs lbs "/2 Alpha TMT A As-recei\'ed Control 3250 3870 24.7

I 650 I 300 5 15 1600 2190 42.9 2 650 l 200 5 15 1960 2170 38.3 3 750 1 300 5 15 1875 2150 35.5 4 750 1 200 5 15 1930 2160 35.1 5 825 .5 300 5 15 1825 2210 31.8 6 825 .5 200 5 15 1870 2130 32.7 7 1010 300 5 15 1-180 2150 22.1

TABLE [1 Continued TENSILE PROPERTIES AT 300C FOR TMT OF ANNEALED TUBING Tensile Properties" Speci- Anne-.1] Parameters Deformation Parameters Tom] men Temperature Time Temperature Ci Hold Time 0.271 YS UTS Elongation No. "C Hour C Elongation Minutes lbs lbs 9? 8 I010 .I67 300 5 l5 1700 2I70 21.2 9 Il0 .167 200 15 I750 2240 23.3 750 I I050 2I30 37.7 I l 750 I 200 /3 I725 2200 36.] I2 750 I 200 5/5 15 2 l 2200 "I 1.0 13 750 l 200 5 60 I8l0 2080 35.7 14 750 I 200 3/3/3 15 I925 2l64 34.7 I5 300 L 3280 3825 26. lo 750 I 200 5/5/5 I5 2225 2230 24.3 17 750 l 200 3/3/3/3 I5 2080 2120 9.5 18 As-rcceived 200 3/3/3 I5 2680 3820 22.9

Beta TMT 32 IOI0 .167 I000 I/3 I260 2I30 8.0

"For reference onl data from Table I. Specimen "Sec non.- (all. 'l'aihlc I.

"Hold Time is 15 minutes for each tlel'ornmtion in the two sequential dcliirmulions 011 each. Sample was cooled in air to room temperature between deformations.

Several things are apparent from this table. First, annealing in the beta range (10lOC) gave decidedly inferior results, whether followed by alpha TMT (specimens 7, 8 and 9) or beta TMT (specimen 32). Second, annealing in the upper alpha range without subsequent TMT (specimen No. 10) gave a marked increase in ductility, as indicated by the total elongation, but a great decrease in yield and ultimate strengths. The decrease in yield strength is particularly marked and particularly important. Third, annealing in the intermediate alpha range (Specimen No. 15) had virtually no effect on properties.

Fourth, deformation of upper alpha annealed samples at 200300C up to a total of about 10 percent greatly increased the yield strength as compared to the annealed material without reduction in ductility. Fifth, excessive deformation (samples 16 and 17) caused reduction in ductility as compared to the optimum range.

Electron microscope photographs were made of the alpha annealed material before and after the TMT. Before the TMT, it exhibited the microstructure of well annealed material. There were precipitates within the grains similar to those mentioned above. While these could serve as dislocation sources, no dislocation structure was evident in the annealed material. After the TMT, a moderate but quite uniform dislocation array was shown. A rather complex network had formed, indicating considerable dislocation activity. There were numerous small loops, probably formed from some type of dipole formation. There was also a hint at high (100,000 X) magnification of dislocation interaction with very fine precipitates.

Specimens in the TMT condition (corresponding to Specimen 14, Table II) and in the as-received condition were aged 1000 hours at 300C to determine their relative thermal stabilities. No effect of this exposure was observed on tensile properties at room temperature or 343C for metal in either condition, indicating that, at least for this thermal exposure, the TMT-produced microstructures is as stable as the as-received.

Corrosion properties of TMT tubing were compared with as-received and annealed specimens. The corrosion tests were conducted in an autoclave at 400C and 500C at a steam pressure of 1500 psig (pounds per square inch gauge). The 400C test exposures were for three days and fourteen days to investigate the response of the TMT materials to standard ASTM corrosion tests. The 500C tests are conducted to characterize possible influences of the TMT treatments on the high temperature failure characteristics of the zircaloy tubing.

The corrosion resistance of the TMT tubing was found to be essentially the same as that'of the asreceived tubing, which met specifications for use as cladding for oxide-type nuclear fuel. It differed but little from undeformed annealed material.

EXAMPLE II A inch length of zircaloy-2 tubing of the type described in Example I, which had been heated at 750C for 1 hour and allowed to cool in air was held vertically in articulating grips in an insulated steel pipe suspended from a ceiling crane. It was heated by steam at 200C and 250 psig generated in an autoclave and sent through the top of the pipe to condense inside and outside the tube with condensate returning to the autoclave. The tube was loaded by lead bricks on a pan secured by grips to the lower end of the tube. Temperatures, load versus time (strain rate) and elongation were monitored using the test.

In more detail, lead bricks were placed in the pan in sufficient quantity to give a total maximum load on the tube of 2,000 pounds. The pan initially rested on the floor. The tube was pulled by its upper end with a force regulated so as to give a total elongation of about 0.1 inch/minute (e.g., a strain rate of 0.001 inch/inch/minute). In about 50 minutes the tube had elongated about 5 inches and the load had reached its maximum of 2,000 pounds, the pan being lifted from the floor. The assembly was then allowed to hang for 15 minutes, during which the tube increased in length by about another inch.

The results were as follows:

a. Temperature control Center: 393 i 5F (201 2*: 3C) Vertical gradient between center 103 inches Tube 1: :t 10F prior to and during loading Tube 2: t 5F prior to and during loading b. Deformation Two tubes processed Two (2) successful TMTs C. Post-TMT dimensional evaluation Tube 1 rough measurements showed center 87 inches with OD uniformity to i 0.001 inch.

Tube 2 Measurements (every 3 inches along tube) showed center 99 inches with OD uniformity of: 0.0008 inch and with 1D uniformity ofi 0.0007 inch.

d. Post-TMT Tensile Properties (Tube 2) Room temperature properties (average of 6 tests from specimens evenly spaced along uniform tube region of Tube 2) kilopounds per square inch. The symbols YS, UTS and TE stand for yield strength. ultimate tensile strength and tensile elongation, respectively. 7

For comparison, the specifications for this coldworked and stress relieved tubing as supplied by the vendor called for the following minimum value:

0.2 YS (ksi) UTS (ksi) TE (70) It will be seen that the ductility of the treated tubing greatly exceeded the specification requirements, while the yield strength and ultimate strength equaled or exceeded those requirements.

The American Society for Testing Materials Specification B353-7l, Wrought Zirconium and Zirconium Alloy Seamless and Welded Tubes for Nuclear Service, specifies the following requirements for fully annealed tubing, of a composition corresponding to zircaloy-2, at room temperature.

0.27: YS (ksi) UTS (ksi) TE (75) The treated tubing greatly exceeded the requirements for fully annealed tubing in ductility as well as in strength.

EXAMPLE lll Tubing of the same type as that described in the previous example, but from a different batch supplied by the vendor, was treated in the manner described in Example ll, except that the center temperature during the deformation was maintained at 225C. Tensile tests were run on both as-received and on thermomechanically treated (TMT) tubing. Results are shown in Table 111.

While the specific examples are directed to zircaloy- 2, the process is also applicable to other similar alloys, e.g., zircaloys -1, -3 and -4. It is also applicable to other zirconium alloys comprising at least 95 percent zirconium and one or more constituents Tests) (No as-received test at 382C) Room Temperature Tubes from same vcndors lot but processed separately in TMT.

selected from the group consisting of chromium, tin, miobium, iron, tantalum, nickel and aluminum. It is particularly advantageous in those situationsin which the heat treatment produces precipitates with which the TMT-produced dislocation structure interacts. Such a heat treatment is usually applied after the deformation process to allow the precipitates to nucleate and grow upon the dislocation; however, precipitation may occur during the deformation process.

I claim:

1. A method of treating a metal comprising at least percent zirconium and also comprising at least one constituent selected from the class consisting of chromium, tin, niobium, iron, tantalum, nickel and aluminum, which metal is in a thoroughly annealed condition, said method comprising subjecting said metal to a tensile creep strain of 3 to 10 percent while holding it in a temperature range in which creep will occur, yet which is below the temperature for significant recovery, said tensilecreep strain being attained by tensileloading said metal at a strain rate of about 0.001 to 0.01 inch/inch/minute until said strain is attained, followed by holding said metal at the final load for at least 15 minutes.

2. A method as defined in claim 1 wherein said metal comprises at least 97.5 percent zirconium and also comprises tin and said temperature range is 200 to 350C.

3. A method as defined in claim 2 wherein said metal is zircaloy-2.

4. A method as defined in claim 1 wherein said tensile creep strain in applied in increments of 3 to 5 percent.

5. A method as defined in claim 4 wherein the metal is allowed to cool to substantially room temperature between successive incremental strains.

6. A method as defined in claim 1 wherein said treatment consists in applying a single tensile creep strain of 3 to 5 percent and then allowing the metal to cool to room temperature.

7. A method as defined in claim 3, wherein said method consists in applying a single tensile creep elongation of substantially 5 percent at a temperature of substantially 225C and then allowing the metal to cool to room temperature.

8. A method as defined in claim 2, wherein said method consists in applying a tensile loading at a temperature of substantially 225C. until a strain of substantially 5 percent is attained.

9. A method of treating tubing formed of a metal comprising at least 95 percent zirconium and also comprising at least one constituent selected from the class consisting of chromium, tin, niobium, iron, tantalum, nickel and aluminum, said tubing being formed by a series of cold working steps followed by a final stress relief, said method comprising the steps of (a) heating said tubing at a first temperature in the upper part of the alpha range of zirconium until the metal is in a thoroughly annealed condition, and (b) subjecting said tubing while in said annealed condition to a tensile creep strain of 3 to 10 percent while holding it at a second temperature in a range in which creep will occur yet which is below the temperature for significant recovery, said tensile creep strain being attained by tensile loading said metal at a strain rate of about 0.001 to 0.01 inch/inch/minute' until said strain is attained, fol- 13. A method as defined in claim 12 wherein said tensile creep strain is applied in increments of 3 to 5 percent.

14. A method as defined in claim 13 wherein the tubing is allowed to cool to room temperature between successive incremental strains.

15. A method as defined in claim 12 wherein the tensile creep strain consists of a single tensile creep elongation of 3 to 5 percent.

16. A method as defined in claim 11 wherein the tensile creep strain consists of a single tensile elongation of substantially 5 percent and is carried out at a temperature of substantially 225C.

17. A method as defined in claim 11 wherein said tubing is zircaloy-Z tubing that has been formed by a series of cold-working steps with a final stress relief in the range of 418C to 538C.

18. A method as defined in claim 17 wherein the tensile creep strain is carried out by applying a tensile,

elongation at a temperature of substantially 225C. until a strain of substantially 5 percent is attained. 

1. A METHOD OF TREATING A METAL COMPRISING AT LEAST 95 PERCENT ZIRCONIUM AND ALSO COMPRISING AT LEAST ONE CONSTITUENT SELECTED FROM THE CLASS CONSISTING OF CHROMIUM, TIN, NIOBIUM, IRON, TANTALUM, NICKEL AND ALUMINUM, WHICH METAL IS IN A THOROUGHLY ANNEALED CONDITION, SAID METHOD COMPRISING SUBJECTING SAID METAL TO A TENSIL CREEP STRAIN OF 3 TO 10 PERCENT WHILE HOLDING IT IN A TEMPERATURE RANGE IN WHICH CREEP WILL OCCUR, YET WHICH IS BELOW THE TEMPERATURE FOR SIGNIFICANT RECOVERY, SAID TENSIL CREEP STRAIN BEING ATTAINED BY TENSILE LOADING SAID METAL AT A STRAIN RATE OF ABOUT 0.001 TO 0.01 INCH/INCH/MINUTE UNTIL SAID STRAIN IS ATTAINED, FOLLOWED BY HOLDING SAID METAL AT THE FINALL LOAD FOR AT LEAST 15 MINUTES.
 2. A method as defined in claim 1 wherein said metal comprises at least 97.5 percent zirconium and also comprises tin and said temperature range is 200* to 350*C.
 3. A method as defined in claim 2 wherein said metal is zircaloy-2.
 4. A method as defined in claim 1 wherein said tensile creep strain in applied in increments of 3 to 5 percent.
 5. A method as defined in claim 4 wherein the metal is allowed to cool to substantially room temperature between successive incremental strains.
 6. A method as defined in claim 1 wherein said treatment consists in applying a single tensile creep strain of 3 to 5 percent and then allowing the metal to cool to room temperature.
 7. A method as defined in claim 3, wherein said method consists in applying a single tensile creep elongation of substantially 5 percent at a temperature of substantially 225*C and then allowing the metal to cool to room temperature.
 8. A method as defined in claim 2, wherein said method consists in applying a tensile loading at a temperature of substantially 225*C. until a strain of substantially 5 percent is attained.
 9. A method of treating tubing formed of a metal comprising at least 95 percent zirconium and also comprising at least one constituent selected from the class consisting of chromium, tin, niobium, iron, tantalum, nickel and aluminum, said tubing being formed by a series of cold working steps followed by a final stress relief, said method comprising the steps of (a) heating said tubing at a first temperature in the upper part of the alpha range of zirconium until the metal is in a thoroughly annealed condition, and (b) subjecting said tubing while in said annealed condition to a tensile creep strain of 3 to 10 percent while holding it at a second temperature in a range in which creep will occur yet which is below the temperature for significant recovery, said tensile creep strain being attained by tensile loading said metal at a strain rate of about 0.001 to 0.01 inch/inch/minute until said strain is attained, followed by holding said metal at the final load for at least fifteen minutes.
 10. A method as defined in claim 9 wherein said metal comprises at least 97.5 percent zirconium and also comprises tin, said first temperature is in the range 650* to 850*C. and said second temperature in the range 200* to 350*C.
 11. A method as defined in claim 10 wherein said metal is zircaloy-2.
 12. A method as defined in claim 9 wherein said strain is a unidirectional tensile creep elongation.
 13. A method as defined in claim 12 wherein said tensile creep strain is applied in increments of 3 to 5 percent.
 14. A method as defined in claim 13 wherein the tubing is allowed to cool to room temperature between successive incremental strains.
 15. A method as defined in claim 12 wherein the tensile creep strain consists of a single tensile creep elongation of 3 to 5 percent.
 16. A method as defined in claim 11 wherein the tensile creep strain consists of a single tensile elongation of substantially 5 percent and is carried out at a temperature of substantially 225*C.
 17. A method as defined in claim 11 wherein said tubing is zircaloy-2 tubing that has been formed by a series of cold-working steps with a final stress relief in the range of 418*C to 538*C.
 18. A method as defined in claim 17 wherein the tensile creep strain is carried out by applying a tensile elongation at a temperature of substantially 225*C. until a strain of substantially 5 percent is attained. 